The invention relates to a process for producing a product gas, such as syngas or an unsaturated hydrocarbon, utilizing a combination of an exothermic partial oxidation reaction and an endothermic steam reforming reaction. More particularly, the oxygen for the exothermic reaction is received by transport through an oxygen selective ion transport membrane element, and the heat generated by the exothermic reaction is supplied to the endothermic reaction.
Natural gas and methane, a major constituent of natural gas, are difficult to economically transport and are not easily converted into liquid fuels, such as methanol, formaldehyde and olefins, that are more readily contained and transported. To facilitate transport, methane is typically converted to synthesis gas (syngas) which is an intermediate in the conversion of methane to liquid fuels. Syngas is a mixture of hydrogen and carbon monoxide with H2/CO molar ratio from about 0.6 to about 6.
One method to convert methane to syngas is steam reforming. The methane is reacted with steam and endothermically converted to a mixture of hydrogen and carbon monoxide. The heat sustaining this endothermic reaction is provided by external combustion of fuel. The steam reforming reaction is of the form:
CH4+H2Oxe2x86x923H2+CO.xe2x80x83xe2x80x83(1)
In a partial oxidation reaction, methane is reacted with oxygen and converted to syngas in an exothermic reaction. The partial oxidation reaction is of the form:
CH4+xc2xdO2xe2x86x922H2+COxe2x80x83xe2x80x83(2)
Both the steam reforming reaction and the partial oxidation reaction are expensive to maintain. In steam reforming, a significant quantity of fuel is required to provide the heat to sustain the endothermic reaction. In the partial oxidation reaction, significant energy and capital must be expended to provide the oxygen required to drive the reaction.
U.S. Pat. No. 5,306,411 to Mazanec et al., which is incorporated by reference in its entirety herein, discloses the production of syngas by partial oxidation and steam reforming where the oxygen is obtained by transport through an oxygen selective ion transport membrane element and both reactions take place on the anode or permeate side of the membrane. This membrane element conducts oxygen ions with infinite selectivity and is disposed between an oxygen-containing feed stream, typically air, and an oxygen consuming, typically methane-containing, product or purge stream.
xe2x80x9cOxygen selectivityxe2x80x9d is intended to convey that the oxygen ions are preferentially transported across the membrane over other elements, and ions thereof. The membrane element is made from an inorganic oxide, typified by calcium- or yttrium-stabilized zirconia or analogous oxides having a fluorite or perovskite structure.
At elevated temperatures, generally in excess of 400xc2x0 C., the membrane elements contain mobile oxygen ion vacancies that provide conduction sites for the selective transport of oxygen ions through the membrane elements. The transport through the membrane elements is driven by the ratio of partial pressure of oxygen (PO2) across the membrane: Oxe2x88x92 ions flow from the side with high PO2 to the side with low PO2.
Ionization of O2 to Oxe2x88x92 takes place on the cathode side of the membrane element and the ions are then transported across the membrane element. The Oxe2x88x92 ions then combine to form oxygen molecules or react with fuel while releasing exe2x88x92 electrons. For membrane elements that exhibit only ionic conductivity, external electrodes are placed on the surfaces of the membrane element and the electron current is returned by an external circuit. If the membrane has ionic as well as electron conductivity electrons are transported to the cathode side internally, thus completing a circuit and obviating the need for external electrodes.
The Mazanec et al. ""411 patent discloses contacting an oxygen-containing gas with the cathode side of an oxygen selective transport membrane element. A stream of process gases, such as methane and steam, flows along the anode side of the membrane element. Transported oxygen reacts exothermically with the methane in a partial oxidation reaction forming carbon monoxide and hydrogen. At the same time the heat released by the partial oxidation reaction enables methane and steam to engage in an endothermic reaction to produce additional hydrogen and carbon monoxide. Typically a reforming catalyst is provided to promote this reaction. The syngas can then be converted to methanol or to other liquid fuels by the Fischer-Tropsch process or other chemicals in subsequent processes.
While the Mazanec et al. ""411 patent discloses that a portion of the heat generated by the exothermic partial oxidation reaction may be utilized to maintain the temperature of the ion transport membrane element, no provisions are made for the removal of excess heat from the reactor. Further, while the partial oxidation and steam reforming reactions are best conducted at high pressure, there is no disclosure in Mazanec et al. of a reactor design or sealing configuration to support high pressures.
Commonly owned U.S. patent application Ser. No. 08/848,204, now U.S. Pat. No. 5,820,655 entitled xe2x80x9cSolid Electrolyte Ion Conductor Reactor Designxe2x80x9d by Gottzmann et al. that was filed on Apr. 29, 1997 and is incorporated by reference in its entirety herein, discloses using the heat generated by an exothermic partial oxidation reaction to heat an oxygen-containing feed gas prior to delivery of that feed gas to the cathode side of an oxygen selective oxygen transport membrane element. The U.S. Ser. No. 08/848,204 application also discloses the use of a thermally conductive shroud tube surrounding the membrane elements to enhance the conduction of heat while maintaining isolation of gases. Reactive purge arrangements are disclosed in xe2x80x9cReactive Purge for Solid Electrolyte Membrane Gas Separationxe2x80x9d, U.S. Ser. No. 08/567,699, now U.S. Pat. No. 5,837,125 which was filed Dec. 5, 1995, E.P. Publ. No. 778,069, and also incorporated herein by reference. Both applications are commonly owned with the present application.
U.S. Pat. Nos. 5,565,009 and 5,567,398 to Ruhl et. al., that are incorporated by reference in their entirety herein, disclose manufacturing syngas by steam reforming of methane in a catalyst bed located on the shell side of a tube and shell reactor. The heat for sustaining the reforming reaction is provided by combustion of fuel within tubes where the fuel and oxygen supply (air) are separately heated and only combined after they reach their autoignition temperature. The flow paths of the reactor disclosed by Ruhl et al. are arranged is such a way that the combustion products as well as the endothermic reaction products are cooled before exiting the furnace. The disclosed design allows for the use of lower temperature seals where the combustion tubes are joined to tube sheets.
There remains, however, a need for a reactor for the production of syngas and unsaturated hydrocarbons that utilizes an oxygen-selective ion transport membrane element, is capable of operating at pressures above 150 psig and temperatures in the range of 800xc2x0 C. to 1100xc2x0 C., and has provisions that compensate for dimensional changes in the membrane elements due to thermal heating and due to the uptake and release of oxygen by the membrane elements during operational and transitional periods. The reactor should, in addition, maintain the membrane elements within prescribed temperature limits by careful balance of the heats of reaction and other heat sinks or sources as well as effective transfer of heat from exothermic reactions to endothermic reactions and other heat sinks. It should also increase safety by minimizing the risk of a high-pressure leak of a flammable process or product gas into oxygen containing streams.
It is therefore an object of the invention to provide a process for producing syngas by a process that utilizes both an exothermic reaction and an endothermic reaction wherein the reactions either are balanced or are tailored to generate a slight heat surplus.
A further object of the invention is to control the exothermic reaction and the endothermic reaction by controlling the flow rate, composition and/or pressure of the gases provided to the respective reactions. Such gases include an oxygen-containing feed gas, fuel gases, and steam or carbon dioxide. Further control of the endothermic reaction preferably is achieved by control of localized catalyst activity as well as local control of process gas composition.
Yet another object of the invention is to enable independent control of the exothermic and endothermic reactions by selectively incorporating thermally conductive shroud tubes which separate the reactions while permitting efficient transfer of heat between reactions within the reactor.
A still further object of the invention is to minimize the temperatures experienced by seals as well as to minimize pressures differences for tube-to-tube sheet seals that isolate fuel-containing spaces within the reactor interior. This is accomplished in one embodiment by using a two-stage seal and disposing a buffer gas such as steam at a pressure slightly higher than the process side pressure between the two seals. Thus any leakage through the first stage seal will be steam into the process side and through the second stage seal will be steam into an oxygen containing gas.
In one aspect, this invention comprises a process for producing a product gas in a reactor that contains at least one oxygen selective ion transport membrane element. The oxygen selective ion transport membrane element has a cathode side and an anode side. The process includes the steps of:
(1) flowing in a first direction an oxygen containing gas along the cathode side and transporting a permeate oxygen portion through the oxygen selective ion transport membrane element to the anode side;
(2) isolating both a first process gas and a second process gas from the oxygen containing gas whereby at least the first process gas flows along the anode side and the first process gas is capable of both an exothermic reaction with oxygen and an endothermic reaction with the second process gas;
(3) exothermically reacting the oxygen portion with the first process gas and endothermically reacting the first process gas with the second process gas; and
(4) controlling at least one of the exothermic reaction, the endothermic reaction, and internal heat transfer within the reactor to maintain the oxygen selective ion transport membrane within prescribed thermal limits.
In a preferred embodiment of the first aspect, the exothermic reaction is a partial oxidation reaction and the endothermic reaction is a steam reforming reaction. The oxygen containing gas is air, the first process gas is a light hydrocarbon such as methane or a mixture of light hydrocarbons, hydrogen, and carbon monoxide and the second process gas is either steam or a mixture of steam and carbon dioxide. The first process gas and the second process gas are combined to form a gaseous mixture prior to the exothermic and endothermic reactions.
In another preferred embodiment at least a portion of the anode side of the membrane element is coated with a catalyst layer to accelerate the oxidation reaction between oxygen and a fuel gas at the anode. A catalyst bed is positioned along at least a portion of the anode side of the oxygen selective ion transport membrane. This catalyst is selected to be capable of promoting the endothermic reaction between steam, carbon dioxide and fuel gas. In an alternative preferred embodiment, the second process gas is separated from said first process gas by a thermally conductive, gas impervious member. The first process gas flows through an oxidation passageway and exothermically reacts with permeate oxygen while the second process gas and additional first process gas flow through a reforming passageway.
Preferably, the reforming passageway is packed with a catalyst capable of promoting the endothermic reaction. The local activity of the catalyst bed is selectively tailored to produce a positive balance between the exothermic and endothermic reaction temperatures about a peripheral portion of the bed and a neutral balance in the center of the bed. More preferably, the catalyst activity gradually increases toward the middle and exit end of the bed at a decreasing rate.
In a second aspect of the invention, the process is utilized for-producing a mixture of hydrogen and carbon monoxide (syngas) in a reactor that contains at least one oxygen selective ion transport membrane element. This oxygen selective ion transport membrane element has a cathode side and an anode side. In this second aspect, the said process includes the steps of:
(1) flowing air in a first direction along the cathode side and transporting a permeate oxygen portion through the oxygen selective ion transport membrane element to the anode side;
(2) flowing a gaseous mixture of light hydrocarbons such as methane and steam along the anode side;
(3) exothermically reacting a first portion of the hydrocarbon with permeate oxygen while endothermically reacting a second portion of the hydrocarbon with the steam; and
(4) controlling at least one of the exothermic reaction, the endothermic reaction and internal heat transfer within the reactor to maintain the oxygen selective ion transport membrane at a temperature within prescribed limits.
In a preferred embodiment of this process, the steam is delivered to the reactor at a higher pressure than the methane is delivered to the reactor. By proper positioning of steam and methane inlets, the steam functions as a buffer, to prevent the leaking of flammable methane from the reactor and into oxygen containing spaces within the reactor. Typically, the steam is delivered to the reactor at a pressure that is from 1 to 20 psig greater than the pressure at which the methane is delivered to the reactor.
In a third aspect of the invention, there is provided a reactor that has a hollow shell defining a hermetic enclosure. A first tube sheet is disposed within the hermetic enclosure and defines a first chamber and a second chamber. Within the hermetic enclosure is at least one reaction tube. The reaction tube has a first portion that is fixedly attached and substantially hermetically sealed to the first tube sheet and opens into said first chamber, the remaining portion being axially unrestrained and an oxygen selective ion transport membrane disposed between the first end and the second end of the reaction tube.
In addition, the reactor includes a first process gas inlet for the delivery of a first process gas to the hermetic enclosure at a first pressure, a second process gas inlet for delivery of a second process gas to the hermetic enclosure at a second pressure, an air inlet for delivery of an oxygen containing gas to the hermetic enclosure at a third pressure and a plurality of outlets for the removal of a product gas and reaction by-product gases from the hermetic enclosure.
In a preferred embodiment, the reaction section of the reactor is effective for the selective transport of oxygen from an inside cathode surface thereof to an outside anode surface thereof and an oxidation enhancing catalyst is selectively disposed on the outside surface and a reforming catalyst about the outer surface. The reactor includes at least one sliding seal that engages said reaction tube. The second end of the reaction tube is attached to a floating tube sheet which is part of an internal manifold which is connected to said shell by flexible bellows or by a stuffing box type seal. Alternatively, individual tubes are joined to the floating tube sheet by short flexible bellows.
In another preferred embodiment, the first end of the reaction tube first end is open proximate the first tube sheet and the second end is closed. A feed tube extends within the reaction tube from the open end to a spaced distance from the closed end whereby an outside surface of the feed tube and an inside surface of the reaction tube define a first annulus. Typically, the first annulus has a width that is less than one-half the inside diameter of the feed tube.
In another preferred embodiment, a thermally conductive shroud tube is disposed about an outside surface of the reaction tube and a combination of an inside surface of the shroud tube and an outside surface of the reaction tube define a second annulus. A combination of the thermally conductive shroud and the reaction section define an oxidation passageway and a reforming passageway is disposed on an opposing side of the thermally conductive shroud. This reforming passageway may be packed with a catalyst that is effective in promoting an endothermic steam reforming reaction.
In still another alternative embodiment, the reactor includes a second reaction tube that extends through the hermetic enclosure in generally parallel alignment with the first reaction tube. This second reaction tube also has a first end attached to the first tube sheet and fixed relative to the shell, an opposing second end that is moveable relative to the shell and a second reaction section disposed between the first and second ends. The second reaction tube is open at the first end and closed at the opposing second end. A second feed tube is disposed within the second reaction tube. An outside surface of the second feed tube and an inside surface of the reaction tube define a third annulus.
In a preferred embodiment, the first reaction tube includes an oxygen selective ion transport membrane effective for the selective transport of oxygen from an outer cathode side to an inner anode side and the second reaction tube contains a reforming catalyst. In still another preferred embodiment, the second process gas inlet is disposed between the first tube sheet and the first process gas inlet for delivery of a second process gas selected from the group consisting of carbon dioxide, steam and mixtures thereof to said hermetic enclosure at a second pressure that is greater than the first pressure. A flow restrictor may be disposed between said second process gas inlet and said first process gas inlet.